Method for purifying a fluid

ABSTRACT

A photocatalytic oxidation purification system includes an ultra violet light source and a filter that comprises a pleated wire mesh substrate with a nanophase metal oxide oxidation catalyst suspended on the substrate, wherein the catalyst is applied without an adhesive using an electromechanical plating process. As a fluid containing organic contaminants is directed through the filter in the presence of ultra violet light from the light source, the catalyst oxidizes and decomposes the organic contaminants into environmentally harmless components. Methods of making the purification system including preparing a solution of catalyst and applying the catalyst without adhesive binding material to the filter substrate electromagnetically.

This application is a division of 09/281,011 filed Mar. 30, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to fluid purification and, inparticular, to a method and assembly for the photocatalytic oxidation ofcontaminants within fluid streams.

2. Description of the Related Art

One of today's most pressing societal problems is environmentalpollution. The Earth's atmosphere is contaminated by hazardous organicpollutants such as solvents, alcohols, dyes, and fuel oils. Suchcontaminants pose serious health risks. As a result, purificationsystems are needed to clean air and water to healthy levels in both thehome and the work place. Purification systems also serve to permitcontaminating factories to comply with environmental regulations.

In recent years, photocatalytic oxidation has emerged as a generallyeffective method of purifying fluids such as air and water. This methodinvolves directing a flow of a contaminated fluid into contact with anoxidation catalyst that is simultaneously exposed to electromagneticradiation. Pollutants contained within the fluid are adsorbed onto thesurface of the oxidation catalyst. As a result, the pollutants areoxidized and thereby decomposed into environmentally innocuouscomponents such as water and carbon dioxide. One catalyst that has beenused is anatase titanium dioxide (TiO₂). The absorption of ultra violetlight energy causes the TiO₂ to become highly reactive.

Early photocatalytic oxidation techniques were unsuitable forincorporation into conventional HVAC systems because they were limitedto relatively slow flow rates or were applicable only for liquidflowstreams. For example, U.S. Pat. No. 5,045,288 to Raupp et al.teaches a method wherein a contaminated fluid flows vertically upwardthrough a reaction chamber containing a catalyst bed of loose anatasetitanium dioxide particles resting upon filter paper. Ultra violet lightfrom a source outside of the reaction chamber is exposed onto the outerperiphery of the catalyst bed, producing the desired oxidation ofcontaminants. One limitation of the Raupp system is that the presence ofthe filter paper and the large catalyst bed causes a relatively highpressure loss within the fluid. The filter paper, although sufficientlyporous to permit fluids to slowly flow through it, tends to restrict theflow. Thus, the fluid pressure decreases as it passes through thefilter. Above the filter and downstream thereof, the large catalyst bedfurther restricts the flow, decreasing the pressure even more. Suchpressure loss limits the fluid flowrate and, consequently, the overallrate of purification as well.

Another limitation is that only those catalyst particles on the outeredges of the catalyst bed are exposed to the ultra violet light. Suchlimited surface area of light exposure results in limited overalloxidation, non-uniform oxidation rates within the flowstream, andnon-use of the catalyst particles in the center of the catalyst bed,which are not exposed to the light.

U.S. Pat. Nos. 5,163,626 and 5,308,458 to Urwin et al. teach a methodwherein a flow of contaminated liquid is exposed to ultra violet lightas it flows over a horizontal spinning disc utilized to agitate theliquid. According to a first method, anatase titanium dioxide is mixedwithin the contaminated liquid to produce the desired contact betweenthe contaminants and the catalyst. According to a second method, thecatalyst is coated onto the spinning disc to produce the desiredcontact. The disc is coated by immersing the disc within an aqueoussolution of the catalyst and then baking the disc. The immersion/bakingcycle is repeated 7 to 15 times.

There are several limitations of the Urwin system. One limitation isthat it cannot effectively be used for gas flowstreams, such as air.Regarding the first method, it is difficult if not impossible to mix theanatase titanium dioxide particles within a gas, since the particlesgenerally are not light enough to be carried by the gas. Regarding thesecond method, a gas flowstream is not desirable because most of the gaswill not come into contact with the disc surface. Rather, most of thegas will flow above the disc and avoid being oxidized by the titaniumdioxide. Another limitation of the Urwin system is that it is relativelycomplex and expensive to manufacture. For example, the inlet tubethrough which the liquid flows onto the spinning disc also spins alongwith the disc. The utilization of moving parts makes it more difficultto maintain a leak-free environment and necessitates frequentreplacement of motors and other parts. Another limitation of the firstmethod in particular is that the process necessitates the further stepof filtering the titanium dioxide particles from the purified liquid.Another limitation of the second method in particular is that therepeated immersion/baking process is relatively time-consuming, yetproduces a highly non-uniform coating and a relatively weak bond betweenthe catalyst and the disc.

More recent photocatalytic oxidation methods involve less expensive,passive filters which are more suited for use in conventional HVACsystems. Such filters have relatively large fluid passageways therein sothat a fluid stream may pass through the filter without significantpressure loss. Also, such filters do not have moving parts like thespinning disc of the Urwin system, which may complicate the design andnecessitate frequent replacement of such parts. The filter is typicallymanufactured by coating an adhesive binding material, such as a polymer,epoxy, or other binder, onto an inert substrate. The binding materialmay either be intermixed with a catalyst or the catalyst may be coatedonto the binding material after the latter is applied to the substrate.A limitation of such filters is that ultra violet radiation tends to bumaway the adhesive material, causing the catalyst to flake off of thesubstrate.

One example is U.S. Pat. No. 5,564,065 to Fleck that teaches apurification system comprising a reaction chamber filled with a finefibrous material, such as fiberglass. A powder form of anatase titaniumdioxide catalyst is coated onto the fibrous material by first applyingan adhesive liquid onto the material and then spraying the catalystthereto. The liquid adhesive is applied by immersing the fibrousmaterial in a bath of the liquid adhesive and then removing the fibrousmaterial. As the catalyst is sprayed onto the material, catalystparticles stick to the adhesive liquid coating, forming a layer of thecatalyst on the fibrous material. Ultra violet light is generated by alight source in the center of the chamber.

Similarly, Japanese patent Application No. 10-238799 teaches a filtercomprising an aluminum corrugated honeycomb substrate coated with acolloidal silica type binder containing anatase titanium dioxidecatalyst. The catalyst is first mixed with the binder. The binder isthen coated onto the substrate to form the filter. Ultra violet light isprovided by a nearby light source.

Several characteristics of such filters limit their effectiveness. Onelimitation is that, as mentioned above, the ultra violet light tends tobum away the adhesive material that carries the catalyst. In operation,these filters lose catalyst particles relatively quickly and must befrequently replaced. Another limitation is that use of an adhesivebinding material often results in an uneven coating of the catalyst onthe filter, resulting in a waste of unusable catalyst. Anotherlimitation is balancing the need to maximize contaminant contact withthe catalyst with the need to minimize pressure drop access the filter.Prior art photocatalytic oxidation systems do not strike the balancevery well, resulting in either high contact and high pressure drop orlow pressure drop, but low contact. A further limitation is that withsome systems only a very limited surface area of the catalyst is exposedto the ultra violet light, resulting in a lower overall oxidation rate,non-uniform oxidation, and waste of the unexposed catalyst.

Although prior art photocatalytic oxidation systems are generallyeffective, there is a need to improve purification rates by providingincreased catalyst surface contact area and by increasing the portion ofthe catalyst surface that is directly exposed to electromagneticradiation. There is also a need to increase system efficiency byminimizing the loss of catalyst through general use, resulting in lessshutdowns for filter replacement.

SUMMARY OF THE INVENTION

Accordingly, it is a principle object and advantage of the presentinvention to overcome some or all of the limitations of the prior artand to provide an improved photocatalytic oxidation filter with a higherrate of purification.

The present invention comprises a durable, flexible, long-lastingphotocatalytic oxidation filter with a substantially uniform,strongly-bonded, non-adhesive, coating of an oxidation catalyst thereon,as well as a method of manufacturing and using a photocatalyticoxidation filter, to improve upon purification rates and improvepurification efficiency by minimizing the loss of the catalyst duringgeneral use. The present invention provides a filter configured to havea relatively large surface contact area of the catalyst withoutsignificantly limiting the portion of the catalyst surface that isdirectly exposed to a source of electromagnetic radiation and withoutcausing a high pressure drop.

In accordance with one embodiment, the present invention comprises anassembly for decomposing organic components within a fluid where theassembly directs the fluid into contact with an oxidizing catalyst thatis exposed to electromagnetic radiation. The assembly comprises a filterwith an oxidizing catalyst suspended there on, a fan for directing thefluid into contact with the catalyst, and an electromagnetic radiationsource adapted to emit electromagnetic radiation onto the filter as thefluid is flowing through the filter. The filter comprises a pleated,wire mesh screen formed from a transition metal and an oxidationcatalyst electromagnetically coated onto the screen. The oxidizingcatalyst comprises metal oxide such as nanophase anatase titaniumdioxide and has the property of accelerating the oxidation of theorganic components when the oxidizing catalyst is excited by theelectromagnetic radiation. The filter is configured to efficientlypurify the organic compounds within the flow without substantiallyimpeding the flow.

In accordance with another embodiment, the present invention comprises amethod of manufacturing a purification system for decomposing organiccontaminants within a fluid. The method comprises a step of providing asource of electromagnetic radiation and a step of providing a filter inwhich the filter comprises a pleated substrate formed from a transitionmetal with a nanophase metal oxide catalyst suspended on the substrate.The step of providing a filter comprises the steps of mixing a metaloxide catalyst with a solvent, such as water, to form a solution,applying ultrasonic or mechanical energy to the solution, etching thesubstrate, and applying the catalyst solution to the etched substrateusing an electromechanical process. In one embodiment, the step ofapplying the catalyst comprises adding a wetting agent to the solution,adding a polar solvent to the solution, adding an electricallyconductive solvent to the solution to make the solution electricallyconductive, providing a power source having a positive terminal and anegative terminal, electrically connecting one or more anodes to thepositive terminal of the power source, submerging the anodes within thesolution, electrically connecting the substrate to the negative terminalof the power source so that the substrate forms a cathode, submergingthe substrate within the solution, and baking the substrate at atemperature greater than about 150° F. During the step of submerging thesubstrate within the solution, the power source applies a negativecharge onto the substrate and generates a current within the solution,whereby positive ions of the catalyst are coated onto the substrate,forming a high strength bond between the catalyst and the substrate.

In accordance with yet another embodiment, the step of applying thecatalyst comprises the steps adding a wetting agent to the solution,adding a polar solvent to the solution, applying a negative charge ontothe substrate, spraying the solution through a positively charged needleonto the substrate, and baking the substrate at a temperature greaterthan 150° F. Positively charged ions of the catalyst form within thesolution as the solution flows through the positively charged needle.The ions become attracted to the negatively charged substrate to form ahigh strength bond between the catalyst and the substrate.

All of these embodiments are intended to be within the scope of theinvention herein disclosed. These and other embodiments of the presentinvention will become readily apparent to those skilled in the art fromthe following detailed description of the preferred embodiments havingreference to the attached figures, the invention not being limited toany particular preferred embodiment(s) disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a photocatalytic oxidation purificationsystem in accordance with the present invention;

FIG. 2 is a perspective view of the photocatalytic oxidation filter ofFIG. 1;

FIG. 3A is a magnified sectional view of a surface of the filtersubstrate with one embodiment of a nanophase oxidation catalyst coatedthereon;

FIG. 3B is a magnified sectional view of a surface of the filtersubstrate with an alternative embodiment of the oxidation catalystcomprising a hybrid microphase-nanophase catalyst coated thereon;

FIG. 4 is a schematic view illustrating one embodiment of a method ofthe present invention of electromagnetically plating the catalyst ontothe filter substrate; and

FIG. 5 is a schematic view illustrating an alternative embodiment of amethod of the present invention of electromagnetically plating thecatalyst onto the filter substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the accompanying figures, various embodiments of thepresent invention may be explained in more detail. With reference firstto FIG. 1, the present invention comprises a photocatalytic oxidationpurification system filter system 10 for purifying a fluid. Thepurification system 10 operates to effectively remove substantially allof the organic contaminants in a gaseous or liquid flowstream. Thesystem 10 comprises a housing 12 with a fluid intake vent 14 and a fluidoutlet vent 16 wherein the housing encloses means 18 for directing fluidfrom the intake vent 14 to the outlet vent 16. The housing 12 alsoencloses an oxidation catalyst filter 20 comprising a substrate 22supporting catalyst thereon, a source of electromagnetic radiation 24,an optional reflector 26 and an optional means for pre-filtering thefluid 28. In operation, fluid 30 is directed by the directing means 18from the ambient through the intake vent 14, through the optionalpre-filter means 28, through the oxidation catalyst filter 20, and outthrough the outlet vent 16. By the time the fluid drawn into the intakevent 14 has passed the oxidation catalyst filter 20, substantially allof the organic contaminants present in the fluid are oxidized andremoved.

In one embodiment of the present invention, the directing means 18comprises a blower for mechanically directing fluid through thepurification system 10, although any means for directing fluidefficiently within the housing 12 is contemplated. The source ofelectromagnetic radiation 24 comprises an elongate light bulb that whenenergized radiates energy in a range that includes ultra-violet light,although energy radiated in a wider or different range ofelectromagnetic energy may be effective. When activated, theelectromagnetic energy radiates and contacts the filter directly. Ifdesired, a reflector 26 may be positioned within the housing 30 whereinthe reflector is configured such that the radiation directed toward thereflector 26 from the light bulb bounces back and hits the catalystfilter 20 indirectly. In one embodiment of the present invention, theoptional pre-filter means 28 comprises a 5 micron pre-filter 28 a and aHEPA filter 28 b.

As contemplated, contaminants within the flow 30 are adsorbed onto thesurface of the substrate 22 that makes up the catalyst filter 20. Theoxidation catalyst on the catalyst filter is simultaneously excited byenergy received from the electromagnetic radiation source and decomposesthe adsorbed contaminants into environmentally innocuous compounds. Forexample, carbon monoxide, a toxic compound, is decomposed into water andcarbon dioxide. Similarly, many other organic compounds, both simple andcomplex, such as alcohols, solvents, dyes, and fuel oils, may bedecomposed into harmless compounds.

The substrate 22 preferably has a plurality of orifices large enough topermit the fluid 30 to pass through the catalyst filter 20 withoutsignificant pressure loss. With reference to FIG. 2, the substrate 22preferably comprises a wire mesh screen, although it may be of anyconfiguration that minimizes the pressure drop across the filter whilesimultaneously providing a large surface area of contact between thecatalyst on the filter and the contaminants in the fluid. The wire meshconsists of a plurality of cells formed from the junction of each wirein the mesh. In this case, the cells are generally square-shaped so asto permit the passage of fluid therethrough in such a manner thatcontaminants with the fluid are more likely to come into contact withthe walls of the cells (i.e., the wires) as fluid passes therethrough.The generally cylindrical cross-section of the wires in the mesh is alsoadvantageous in that it permits better interface between the catalystsupported on the wires and the fluid contaminants. Of course, the cellsof wire mesh may come in all sizes. It is preferred that the cell sizebe sufficiently small, but not so small as to impede the flow of fluid.It is contemplated that the substrate be of any configuration havingopenings forming cells through which an impure fluid may pass. It isimportant that the walls of the cells maximize contact of catalystsupported thereon with contaminants within the fluid. Certain honeycombdesigns of the prior art incorporated openings (cells) that were toolarge to effectuate sufficiently high contact between the substratecells and the contaminants. It is also important that the substrate 22have sufficient strength and flexibility to permit a strong bond withthe catalyst and withstand significant flows of contaminated fluidtherethrough. Preferably, the substrate is made of a transition metalsuch as aluminum, although other transition metals may be effective aswell. To further. enhance contact between the catalyst supported on thesubstrate 22 and the contaminants in the fluid 30, the substrate 22 ispreferably pleated to minimize the volume of fluid that does notphysically contact the substrate 22 as its passes therethrough. Thepleats of the substrate 22 may have any particular shape orconfiguration provided the substrate is substantially not flat. Bypleating the substrate, the centerline of the substrate cells may bepositioned at an angle relative to the direction of flow of the fluid.The pleated configuration also generates turbulence within the fluidflow, limiting the possibility that a portion of the fluid would passthrough the filter without coming into contact with the catalyst. Thispermits the system to achieve faster purification and to accommodatehigher fluid flow rates. In that regard, the present invention is asignificant improvement over prior art filter substrates that, in orderto minimize pressure drops, incorporated fluid cells or openings thatwere either very large or were not angled relative to the direction offlow. The present invention is also a significant improvement in that itis made of a material that bonds effectively with the catalyst withoutan adhesive, as explained below, and can withstand high flow rateswithout collapsing.

The light source 24 provides light energy to the filter 20. Faster andmore effective oxidation of contaminants has been achieved by usingultra violet light having a wavelength in the range of about 200-400nanometers. When used in combination with a pleated substrate 22, thelight source 24 is preferably tubular in shape and has a lengthsubstantially equal to the length of the substrate 22, although a lightsource shorter or longer may still be effective. This configuration doesnot significantly impede fluid flow through the system, and it ensuresthat substantially all of one side of the pleated substrate will bedirectly exposed to light 26. Other shapes of the light source couldalso be utilized, keeping in mind these goals. The light source 24 maybe provided either upstream or downstream of the filter 20.Alternatively, multiple light sources could be provided so that bothsides of the filter 20 are exposed to the light.

Ultra violet light rays are unidirectional, which means that any surfacethat is not directly exposed to the light source will not receive anylight energy therefrom. Purification can be improved if a larger portionof the catalyst surface is directly exposed to the light. Therefore, thetubular light source 24 is preferably oriented perpendicular to thedirection of the pleats in the substrate 22. This ensures thatsubstantially all of the surface area of the filter, on one side, isdirectly exposed to the light 26 emitted from the light source 24.Alternatively, the light source and filter may be positioned so that thelight source 24 is oriented parallel to the direction of the pleats ofthe filter 20. In that arrangement, it may be desirable to bow thefilter so that its entire surface is exposed to the light source. It ispossible, however, to orient the light parallel to the pleats withoutbowing the substrate and still ensure that the entire surface of thesubstrate facing the light will be exposed. The angle of the pleats mayeffect whether the entire side is exposed.

As alluded to above, oxidation rates may be improved by providing somemeans of utilizing more of the light emitted from the light source 24.In one embodiment, the reflector 26 is used to indirectly radiate energyback to the catalyst filter 20. Alternatively, one or more filters maybe provided in a manner surrounding the light source so thatsubstantially all of the light may be utilized for purification.

Regarding the catalyst, a preferred catalyst is anatase titanium dioxide(TiO₂). Alternative catalysts may be used, however, including zirconiumoxide (ZrO₂), antimony oxide (Sb₂O₄), zinc oxide (ZnO), stannic oxide(SnO₂), cerium oxide (CeO₂), tungsten oxide (WO₃), and ferric oxide(Fe₂O₃). To maximize the effectiveness at purifying the fluid, thepreferred embodiment of catalyst for the present invention is nanophaseanatase TiO₂; i.e., particles having a diameter as small asapproximately 10⁻⁹ meters, to maximize the number of “receptor sites”where the catalyst gives up electrons to oxidize pollutants. As thenumber of receptor sites of the catalyst is increased, the oxidationrate rises. For anatase titanium dioxide, the inventor understands thata 50% reduction in average particle size would increase the oxidationrate by a factor of twelve. Thus, a significant particle size reductioncan improve oxidation rates by orders of magnitude.

Referring now to FIG. 3A, it can be appreciated that the presentinvention contemplates that the entire surface of the substrate, whichin the preferred embodiment is wire mesh 32 a, is coated with ananophase catalyst 34 a, preferably a metal oxide. A filter coated witha nanophase metal oxide catalyst without the use of adhesive bondingadvantageously provides faster oxidation than existing photocatalyticoxidation filters. It is contemplated that faster and more efficientoxidation may be achieved, however, by increasing the fluid contactsurface area of the nanophase catalyst even more. A larger contactsurface area of the nanophase catalyst coating may be produced bycoating a mixture of microphase and nanophase catalyst particles ontothe substrate 22, where microphase catalyst comprises particles having adiameter of the magnitude of 10⁻⁶ meters. FIG. 3B shows a cross-sectionof the surface of the substrate, preferably a wire mesh 32 b, with amixture 34 b of microphase and nanophase anatase titanium dioxide aremixed together. The mixture may be prepared by applying a solution ofmicrophase and nanophase catalyst to the substrate in the same manner asa solution of nanophase catalyst is applied to the substrate, asexplained further below. The solution of microphase and nanophasecatalyst is prepared by adding microphase catalyst and nanophasecatalyst to a container of solvent and blending the solution with anosterizer having high speed impellers. During the blending process, thelarger microphase particles 36 act to shear the smaller nanophaseparticles 38 down to their smallest particle size. In the process, thenanophase particles 38 become coated onto the larger microphaseparticles 36. Thereafter, the microphase-nanophase catalyst mixture maybe applied to the substrate 22 according to one of the manufacturingmethods described below.

Advantageously, this process greatly increases the contact surface areaof the nanophase particles 38, improving oxidation rates and permittinglarger flow rates of fluid through the filter. For a pleated wire meshsubstrate 22 having a surface area on one side of about 80 in², flowrates of about 270 ft³/minute can be accommodated. A further advantageof mixing microphase and nanophase catalyst particles together is thatit results in a stronger, more durable coating of the catalyst onto thesubstrate 22. This is because the nanophase particles reside within theinterstices between the microphase particles, thereby strengthening themicrophase layer. This enhances the ability of the catalyst to remain onthe substrate 22 even when the substrate is flexed. Sufficient strengthand durability of the catalyst coating have been achieved by utilizing acatalyst comprising at least about 30% nanophase particles. From abalance of economy and effectiveness, a catalyst mixture comprisingabout 70% microphase particles and about 30% nanophase particles ispreferred given the less expensive cost of microphase catalyst ascompared to nanophase catalyst.

A nanophase catalysts such as nanophase anatase titanium dioxide isavailable in powder form from Nanophase Technologies Corporation ofIllinois for about $55/lb. The average particle size of such nanophaseanatase titanium dioxide available from Nanophase Technologies is about30 nanometers. Microphase catalysts can be acquired from a variety ofsources. One powder form of microphase anatase titanium dioxide that issuitable for the embodiments described above is Kronos 1000, sold byKronos for about $5/lb. This substance has an average particle size ofabout 0.2 microns. Those in the art will understand that thesesubstances are exemplary and that other substances having differentparticles sizes may be utilized, keeping in mind the goals of rapidoxidation of organic contaminants and producing a more durable,long-lasting coating of the catalyst onto the substrate 22.

The present invention also comprises a method of manufacturing apurification system that comprises the steps of providing a catalystfilter having qualities and benefits substantially as described above.The step of providing a catalyst filter with the capabilities ofsupporting a nanophase catalyst comprises the steps of etching asubstrate made of a transition metal such as aluminum and preparing acatalyst solution for application to the substrate. The method alsocomprises the step of applying the catalyst solution to the substrate ina manner so as to result in a strong non-adhesive bond between thecatalyst and the substrate. In one embodiment of the present inventivemethod, the step of applying the catalyst solution compriseselectroplating the substrate with the catalyst solution. With referenceto FIG. 4, in one particular embodiment, the electroplating stepcomprises using a power source 44, such as a 12-volt DC battery, havinga positive terminal 46 and a negative terminal 48. The substrate 22 iselectrically connected to the negative terminal 48 of the power source44, which imparts a negative charge onto the substrate. The substrate,therefore, becomes a cathode. One or more electrodes 50 are electricallyconnected to the positive terminal 46 of the power source 44, whichimparts a positive charge onto the electrodes 50. The electrodes 50,thus, become anodes. The substrate 22 is preferably formed from atransition metal, i.e., a metal that easily gives up electrons, such asaluminum or copper. This permits the substrate to more easily conductelectricity in order to form a current within a solution of catalyst andsolvent 52. The catalyst preferably has a high positivity, which allowscatalyst particles to more easily become positively ionized. A preferredcatalyst is anatase titanium dioxide, since it has a very highpositivity.

The substrate 22 and anodes 50 are then submerged within the solution 52containing the catalyst, preferably for about 60-70 seconds. Catalystparticles 54 within the solution 52 lose electrons 56 that are attractedto the positively charged anodes 50. The positively ionized catalystparticles 54 are then magnetically attracted to the negatively chargedsubstrate 22. As a result, the catalyst particles 54 become coated ontothe substrate. The anodes 50 are preferably arranged in a manner so asto surround the substrate 22, which results in a more uniform coating ofthe catalyst thereto. Finally, the substrate is removed from thesolution 52 and baked at a high temperature, preferably about 750° F.,to prevent the catalyst particles from recrystallizing. Advantageously,the baking step effectively freezes the particles in position. It is notnecessary to bake for a long period of time. All that is necessary toprevent recrystallization is to bring the substrate 22 to a hightemperature for a short period.

The step of providing the catalyst solution 52 preferably comprises thesteps of adding a powder form of nanophase catalyst into a bath ofsolvent, such as distilled water, to form a solution. In one preferredembodiment, the catalyst is a mixture of microphase and nanophaseanatase titanium dioxide blended together with mechanical energyprovided by an osterizer or “sandmill”. The latter consists of addingsand to the solution and inserting a spinning disc that circulates thesand within the solution in a turbulent fashion. The sand breaks downthe catalyst to its smallest particles. The sand may then be filteredout, leaving a well mixed solution. It should be noted that puremicrophase or pure nanophase particles could be used. Energy in the formof ultrasound energy, for example, may be applied to the solution 52 bysubmerging a high frequency transducer therein. The ultrasound energybreaks the catalyst particles down to their smallest size. The methodfurther comprises the step of adding a wetting agent to the solution toreduce the surface tension of the solution, which has the benefit ofimproving the uniformity and adherence of the catalyst coating onto thesubstrate 22. A suitable wetting agent for this purpose is TRITON®X-100, sold by J. T. Baker. A polar solvent may then be added todissolve the polar (i.e., organic) wetting agent within the non-polar(i.e., inorganic) water-based solution, to improve the adherence of thecatalyst onto the substrate 22. A suitable polar solvent for thesepurposes is ethyl alcohol. Further, an electrically conductive solventis added to make the solution electrically conductive and to, thus,enable the electroplating process. A suitable conductive solvent ishydrochloric acid. Optionally, a corrosive solvent may also be added,which etches the substrate 22 when the substrate is submerged within thesolution 52. The etching of the substrate produces a rougher surfacethereon, which improves the adherence of the catalyst onto thesubstrate. It also permits the catalyst to mechanically bond to thesubstrate. A suitable corrosive solvent for this purpose is, again,hydrochloric acid. Alternatively, or perhaps additionally, the substrate22 may be etched prior to being submerged within the solution 52, simplyby exposing the substrate to a low pH acid solution.

The above-described electroplating process produces a highly effectivecatalyst filter 20 for use in photocatalytic oxidation systems.Electroplating advantageously produces a highly stable and flexiblefilter, compared to prior art filters manufactured by binding thecatalyst to the substrate with an adhesive or immersing a substratewithin a catalyst solution and then baking it. Filter substratesmanufactured according to the electroplating method identified above canbe repeatedly flexed without causing significant, if ant, catalystparticles to flake off of the substrate.

An alternative method of electromechanically coating the catalyst ontothe substrate 22 comprises electrostatically spraying the catalystsolution 52 onto the substrate. With reference to FIG. 5, alternativesteps of applying a catalyst solution 32 onto the substrate 22 may bedescribed. According to this method, the solution 52 is made asdescribed above, except that there is no need to include an electricallyconductive solvent. A spray gun 58 having a needle 60 is configured tospray the solution 52 under pressure onto the substrate 22. Both theneedle 60 and substrate 22 are preferably formed from a transitionmetal. A very fine spray can be generated by using a needle having asmall orifice, preferably about 0.005 inches in diameter, and byspraying under a pressure within the range of about 1000-1500 psi. Thisimproves the uniformity of the catalyst coating and minimizes waste ofthe catalyst.

A power source 44 having positive and negative terminals 46 and 48,respectively, is also provided. The needle 60 is electrically connectedto the positive terminal 46, which imparts a positive charge onto theneedle. The substrate 22 is electrically connected to the negativeterminal 48, which imparts a negative charge onto the substrate. Thesolution 52 is then drawn into the spray gun 58 and sprayed onto thesubstrate. As the solution 52 flows through the needle 60, catalystparticles within the solution lose electrons that are magneticallyattracted to the positively charged needle. As a result, the catalystparticles become positively ionized as they pass through the needle. Thepositively charged catalyst solution then bonds to the negativelycharged substrate 22, forming a high strength and long-lasting bond.Advantageously, the magnetic attraction between the catalyst and thesubstrate results in a uniform coating. After the catalyst is sprayedonto the substrate, the substrate may be baked at a high temperature toprevent the catalyst from recrystallizing, as described above. Thesolution 32 preferably contains a mixture of microphase and nanophaseanatase titanium dioxide particles blended together with an osterizer asdescribed above. This results in a catalyst coating that is stronger andmore durable than prior art filters, due to the presence of both sizesof catalyst particles, as explained above. The added strength anddurability prevents catalyst particles from flaking off of the substrate22. Further, this method provides these advantages at a relatively lowcost.

Yet another method of coating the catalyst onto the substrate 22 is tosimply spray the solution 52 onto the substrate without applying anelectric charge onto the solution or the substrate. The catalyst coatingmay be less uniform than that produced via the electroplating orelectrostatic spraying methods described above, due to the absence ofelectromagnetic attraction between the solution 52 and the substrate 22.This method is, however, simpler and less expensive.

While certain objects and advantages of the invention have beendescribed herein above, it is to be understood that not necessarily allsuch objects or advantages may be achieved in accordance with anyparticular embodiment of the invention. Those skilled in the art willrecognize that the invention may be embodied or carried out in a mannerthat achieves or optimizes one advantage or group of advantages astaught herein without necessarily achieving other objects or advantagesas may be taught or suggested herein.

Although this invention has been disclosed in the context of certainpreferred embodiments and examples, it will be understood by thoseskilled in the art that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. Thus, it is intended that the scope of the present inventionherein disclosed should not be limited by the particular disclosedembodiments described above, but should be determined only by a fairreading of the claims that follow.

What is claimed is:
 1. A method of decomposing organic components withina fluid, comprising the steps of: providing a filter comprising asubstrate having a plurality of cells and an oxidizing catalystsuspended on said substrate so that said catalyst coats the walls ofsaid cells, said oxidizing catalyst comprising a mixture of nanophasemetal oxide and microphase metal oxide, said mixture formed by addingtogether a substantially nanophase component of metal oxide and asubstantially microphase component of metal oxide, said mixturecomprising at least 30% said nanophase metal oxide, said oxidizingcatalyst having the property of oxidizing said organic components withinsaid fluid when said oxidizing catalyst is exposed to ultra violetlight; generating a flow of said fluid and directing said flow throughsaid filter; and exposing the filter to ultra violet light, said ultraviolet light exciting said oxidizing catalyst to oxidize and therebydecompose said organic components; wherein said substrate is adapted tobe positioned within said flow without substantially impeding said flow.2. The method of claim 1, further comprising forming said substrate froma transition metal.
 3. The method of claim 1, further comprising formingsaid substrate from a pleated wire mesh screen.
 4. The method of claim1, further comprising etching said substrate prior to the application ofsaid catalyst to said substrate, to enhance bonding of said catalystwith said substrate.
 5. The method of claim 1, wherein said oxidizingcatalyst comprises a mixture of nanophase and microphase anatasetitanium dioxide.
 6. The method of claim 1, wherein said fluid comprisesair.
 7. A method of decomposing organic components within a fluid,comprising: generating a flow of fluid containing organic components;bringing said flow of fluid into contact with an oxidizing catalystelectrostatically sprayed and coated onto walls of a plurality of cellsof a substrate without the use of adhesives, said oxidizing catalystcomprising a mixture of a nanophase metal oxide and a microphase metaloxide, said nanophase metal oxide forming at least 30% of said mixture,said oxidizing catalyst having properties so as to be able to decomposeorganic components within a fluid by exposing said fluid to saidoxidizing catalyst in the presence of ultra violet light; and exposingsaid oxidizing catalyst to ultra violet light, said ultra violet lightexciting said oxidizing catalyst to oxidize and thereby decompose saidorganic components.
 8. The method of claim 7, wherein said oxidizingcatalyst is sprayed through a positively charged needle onto saidsubstrate while said substrate is negatively charged.
 9. The method ofclaim 8, wherein said oxidizing catalyst is sprayed under a pressure of1000-1500 psi.
 10. A method of decomposing organic components within afluid, comprising: generating a flow of fluid containing organiccomponents; bringing said flow of fluid into contact with an oxidationcatalyst coated onto walls of a plurality of cells of a substrate, saidoxidation catalyst comprising a mixture of first and second metal oxidesolutions, said first metal oxide solution having substantiallynanophase metal oxide particle size, said second metal oxide solutionbeing substantially formed from a metal oxide having a particle size atleast one order of magnitude larger than nanophase, said first metaloxide solution forming at least 30% of said mixture, said oxidationcatalyst having properties so as to be able to decompose organiccomponents within a fluid by exposing said fluid to said oxidationcatalyst in the presence of ultra violet light; and exposing saidoxidation catalyst to ultra violet light, said ultra violet lightexciting said oxidation catalyst to oxidize and thereby decompose saidorganic components.
 11. The method of claim 10, wherein said substratecomprises a pleated wire mesh screen.
 12. The method of claim 10,wherein said fluid comprises a gas.